This invention relates to optical waveguides. More particularly, it concerns polarized optical waveguides particularly suited for use in single-mode optical communication transmission networks and a method for making such waveguides.
Single-mode propagation of electromagnetic energy at frequencies within the range varying from near infrared to near ultraviolet is known to represent a quantum increase over multi-mode propagation of such energy in the information handling capacity of optical communication systems. The major advantage of single-mode as against multi-mode propagation of light energy lies in the avoidance of signal deterioration due to dispersion caused by differences in velocities between propagated modes in multi-mode systems. In other words, a signal such as a sharp pulse representing a bit of information supplied to one end of a single mode waveguide, will appear as substantially the same pulse at the other end of the same waveguide assuming that propagation is not adversely affected by external perturbations. The same signal supplied to one end of a multi-mode waveguide isolated from external perturbations, however, will appear as a pulse in each of the multiple modes which arrive at the end of a multi-mode waveguide at different intervals of time. Because of the resulting increased length of each transmitted pulse or bit of information, the rate of information transmission in multi-mode waveguides is severely limited.
Single-mode optical waveguide fibers have been formed to provide to some degree pre-established polarization axes to mitigate the effect of external perturbations and for other reasons. Though initially non-circular core configurations of elliptical or rectangular cross-sections were used to obtain limited polarization, more recently compressive lateral stressing of the waveguide core has been advanced as providing a higher degree of polarization. The lateral compressive stress develops birefringence in the waveguide core, causing the index of refraction for light polarized parallel to the applied force to be increased relative to the refractive index for light polarized perpendicular to the direction of the applied force. In this respect, the disclosures of U.S. Pat. Nos. 4,179,189 to Kaminow, 4,354,736 to Maklad, 4,415,230 to Keck, and several of the prior art references cited therein, are exemplary. It is not known, however that the prior art techniques for polarizing optical waveguides have achieved a degree of polarization locking needed to assure that the measure of pre-established or locked polarization will always be sufficiently retained, or exceed the measure of unwanted polarization resulting from external perturbations in a manner to preserve the initial characteristics of a signal transmitted throughout the length of a waveguide subject to such external perturbations.